User talk:ProfKin

December 2015
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Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days.[14] It has no natural satellite. It is named after the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows.[15] Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°.

Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, mass, proximity to the Sun and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the small unknown four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of 735 K (462 °C; 863 °F), Venus is by far the hottest planet in the Solar System, even though Mercury is closer to the Sun. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It may have had oceans in the past,[16][17] but these would have vaporized as the temperature rose due to a runaway greenhouse effect.[18] The water has most probably photodissociated, and, because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind.[19] Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by volcanism.

Venus is one of the four terrestrial planets in the Solar System, meaning that, like Earth, it is a rocky body. In size and mass, it is similar to Earth, and is often described as Earth's "sister" or "twin".[20] The diameter of Venus is 12,092 km (only 650 km less than Earth's) and its mass is 81.5% of Earth's. Conditions on the Venusian surface differ radically from those on Earth because of its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen.[21]

Geography The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. It was finally mapped in detail by Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate there have been some recent eruptions.[22][23]

About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains.[24] Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area.[25]

The absence of evidence of lava flow accompanying any of the visible caldera remains an enigma. The planet has few impact craters, demonstrating the surface is relatively young, approximately 300–600 million years old.[26][27] In addition to the impact craters, mountains, and valleys commonly found on rocky planets, Venus has some unique surface features. Among these are flat-topped volcanic features called "farra", which look somewhat like pancakes and range in size from 20 to 50 km across, and from 100 to 1,000 m high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as "arachnoids"; and "coronae", circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.[28]

Most Venusian surface features are named after historical and mythological women.[29] Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. The latter three features were named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.[30]

The longitudes of physical features on Venus are expressed relative to its prime meridian. The original prime meridian passed through the radar-bright spot at the center of the oval feature Eve, located south of Alpha Regio.[31] After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne.[32][33]

Surface geology Main articles: Geology of Venus and Volcanology of Venus

Maat Mons with a vertical exaggeration of 22.5 Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it has 167 large volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii.[28]:154 This is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years,[34] whereas the Venusian surface is estimated to be 300–600 million years old.[26][28]

Several lines of evidence point to ongoing volcanic activity on Venus. During the Soviet Venera program, the Venera 9 and Venera 10 orbiters obtained optical and electromagnetic evidence of lightning on Venus,[35][36] and the Venera 12 descent probe recorded a powerful clap of thunder soon after it landed.[37] The European Space Agency's Venus Express in 2007 detected whistler waves further confirming the occurrence of lightning on Venus.[38][39] Although rainfall drives thunderstorms on Earth, there is no rainfall on the surface of Venus (though sulfuric acid rain falls in the upper atmosphere, then evaporates around 25 km above the surface). One possibility is that ash from a volcanic eruption was generating the lightning. Another piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold.[40] This may mean that levels had been boosted several times by large volcanic eruptions.[41][42]

In 2008 and 2009, the first direct evidence for ongoing volcanism was observed by Venus Express, in the form of four transient localized infrared hot spots within the rift zone Ganis Chasma,[43][n 1] near the shield volcano Maat Mons. Three of the spots were observed in more than one successive orbit. These spots are thought to represent lava freshly released by volcanic eruptions.[44][45] The actual temperatures are not known, because the size of the hot spots could not be measured, but are likely have been in the 800–1100 K range, relative to a normal temperature of 740 K.[46]

Almost a thousand impact craters on Venus are evenly distributed across its surface. On other cratered bodies, such as Earth and the Moon, craters show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, whereas on Earth it is caused by wind and rain erosion. On Venus, about 85% of the craters are in pristine condition. The number of craters, together with their well-preserved condition, indicates the planet underwent a global resurfacing event about 300–600 million years ago,[26][27] followed by a decay in volcanism.[47] Whereas Earth's crust is in continuous motion, Venus is thought to be unable to sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.[28]

Venusian craters range from 3 km to 280 km in diameter. No craters are smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater.[48] Incoming projectiles less than 50 metres in diameter will fragment and burn up in the atmosphere before reaching the ground.[49]

Internal structure

The internal structure of Venus – the crust (outer layer), the mantle (middle layer) and the core (yellow inner layer) Without seismic data or knowledge of its moment of inertia, little direct information is available about the internal structure and geochemistry of Venus.[50] The similarity in size and density between Venus and Earth suggests they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.[51] The slightly smaller size of Venus suggests pressures are significantly lower in its deep interior than Earth's. The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.[52] Instead, Venus may lose its internal heat in periodic major resurfacing events.[26]

Atmosphere and climate Cloud structure in the Venusian atmosphere in 1979, revealed by observations in the ultraviolet band by Pioneer Venus Orbiter Cloud structure in the Venusian atmosphere in 1979, revealed by observations in the ultraviolet band by Pioneer Venus Orbiter A false-colour image of Venus: ribbons of lighter colour stretch haphazardly across the surface. Plainer areas of more even colouration lie between. Global radar view of Venus (without the clouds) from Magellan between 1990 and 1994 Impact craters on the surface of Venus (image reconstructed from radar data) Impact craters on the surface of Venus (image reconstructed from radar data) Main article: Atmosphere of Venus Venus has an extremely dense atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times that of Earth's atmosphere, whereas the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1 kilometre under Earth's oceans. The density at the surface is 65 kg/m3, 6.5% that of water or 50 times as dense as Earth's atmosphere at 20 °C at sea level. The CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C).[14][53] This makes the Venusian surface hotter than Mercury's, which has a minimum surface temperature of 55 K (−220 °C) and maximum surface temperature of 695 K (420 °C),[54] even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. This temperature is higher than that used for sterilization. The surface of Venus is often said to resemble traditional accounts of Hell.[55][56]

Studies have suggested that billions of years ago the Venusian atmosphere was much more like Earth's than it is now, and that there may have been substantial quantities of liquid water on the surface, but after a period of 600 million to several billion years,[57] a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[58] Although the surface conditions on the planet are no longer hospitable to any Earthlike life that may have formed before this event, it is possible that life exists in the lower and middle cloud layers of Venus.[59][60][61]

Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of the Venusian surface does not vary significantly between the night and day sides, despite the planet's extremely slow rotation. Winds at the surface are slow, moving at a few kilometres per hour, but because of the high density of the atmosphere at the Venusian surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat, pressure and lack of oxygen were not a problem.[62]

Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[63][64] These clouds reflect and scatter about 90% of the sunlight that falls on them back into space, and prevent visual observation of the Venusian surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well lit. Strong 85 m/s (300 km/h) winds at the cloud tops circle the planet about every four to five Earth days.[65] Venusian winds move at up to 60 times the speed of the planet's rotation, whereas Earth's fastest winds are only 10–20% rotation speed.[66]

The surface of Venus is effectively isothermal; it retains a constant temperature not only between day and night but between the equator and the poles.[3][67] The planet's minute axial tilt—less than 3°, compared to 23° on Earth—also minimizes seasonal temperature variation.[68] The only appreciable variation in temperature occurs with altitude. The highest point on Venus, Maxwell Montes, is therefore the coolest point on the planet, with a temperature of about 655 K (380 °C) and an atmospheric pressure of about 4.5 MPa (45 bar).[69][70] In 1995, the Magellan spacecraft imaged a highly reflective substance at the tops of the highest mountain peaks that bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gaseous form to higher elevations, where it is cooler and could precipitate. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).[71]

The clouds of Venus are capable of producing lightning much like the clouds on Earth.[72] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–2007, Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[72] In 2007, Venus Express discovered that a huge double atmospheric vortex exists at the south pole.[73][74]

Another discovery made by Venus Express in 2011 is that an ozone layer exists high in the atmosphere of Venus.[75]

On 29 January 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions."[76][77]